Backyard Aquaponics
A place to discuss aquaponics

Interestng thoughts!

I'm thinking of burying our tank to give the Trout a nice even temp (and to make more space in the greenhouse), I was hoping the tank would be a good thermal mass to help the plants out hadn't really thought enough about the heat transfer from the grow beds though - would they lose lots of heat overnight in the winter (i.e. make the trout too cold) and gain too much heat in summer (making the trout too hot?). It all seems a bit complicated! Can't wait to get going on it all and see what the real results are.

green house and trout are not words id use in the same sentence...............

The ground is only a good insulator if you like the temperature the ground is at!!!!! Otherwise it is a huge heat sink or thermal mass working against you. In ground tanks are a good thing in my area since the average ground temp is quite high and will definitely help me during the few cold nights. For you up in PA, definitely insulate your tank from the ground. If you want to sink it down into the ground that is fine but definitely insulate under and around it if you do.

As to using the tank and grow beds for that matter as thermal mass to help stabilize your greenhouse they will work great. Now for heating things up quicker in the winter, perhaps a black painted exterior to the tank where the sun will hit would be good but in summer you will probably want to insulate or cover over that with something white or reflective but in truth, it will probably make only minuscule difference what the tank or the beds are made out of. It is the mass of the water and gravel that will store and even out temperatures.

Good luck to you in figuring out how to keep your water warm enough over winter to grow tilapia in your climate.

TCL - do you know what the ground temps get to in PA?

No I don't know the particular ground temps in PA or the area of PA he is in. Dave Donley probably has a better idea. I will venture to say that the average ground temp there is going to be definitely cooler than 70 F though. PA doesn't have the bitter cold the way say Minnesota does but they still get snow and such on a regular basis if not a continuous basis.

Since you are looking at Tilapia, You may want to dig down and insulate around a buried tank.

The ground temperature is the average surface temperature. In PA it is probably in the 50s (F). Not great for Tilapia. The thing is that if you have enough thermal mass, the ground, the tank and the ground in the greenhouse will settle to the average temperature of the air in the greenhouse.

The larger the thermal mass, the less it will fluctuate. The larger the mass, on the other hand, the longer it will take to reach that temperature.

Several years ago, there was experimentation with "earth tempered air" as part of a sustainable living system. The idea was to build a bermed, passive-solar home, and put insulation approximately 3' (1m) below the ground. You also ran pipes to the outside (a minimum of 50' in length) under the ground and an insulative barrier.

The conventional wisdom is that the first year or so, you would be heating the house, but after a year or so, the temperature would move from the average ground temperature to the temperature several degrees higher than that due to the insulation, and the fact that you were heating the huge thermal mass under the insulation blanket.

I would think that this would work similarly. Under the glass of a greenhouse, you would be able to raise the average temperature several degrees so that the average temperature would climb from the 50s to the 70s or even 80s. Again, the larger the thermal mass, the longer it will take to get to the final temperature, and the more stable it will be once it gets there.

I am about two and a half hours north of Dave Donnelly, in Northeastern, PA. We get up to the high 90's (F) in the summer and down into the teens in winter.

These comments are very helpful. Since I am planning to grow tilapia, it makes sense not to bury the tank.

The ideal temp for tilapia is what?

I would keep them around 75F/24C or higher. They breed at 86F/30C. They would start getting unhappy below 70, but then again I haven't try testing what their low temperature is. Ours are Rocky Mountain Whites, but not all of them are white so might have mixed low temperature tolerances.

How about trout? 2c - 20c
you could prolly have them all year round!

+1 Greenedo

I know some people may think it is being picky but please don't refer to dirt being a good or even a moderate insulator. It may be a realatively good insulator but only if you are comparing it with water, steel or some other excellent conductor. Mind you if the ground around your buried tanks or GBs gets wet then you will have an excellent conductor sapping heat from your system.

Having said that it dosn't mean it is not a good idea to bury tanks you just have to consider the properties of your ground and what you are trying to get your system to do.

If people are interested I'm going to post an update on my system about how I've redone the insulation.

viewtopic.php?f=18&t=1353&st=0&sk=t&sd=a&hilit=spc%27s+system&start=120

I have tilapia too and I have been thinking about the Siberian fireplace. Take a huge mass of bricks and channel the vent through it all so that the flue gas from the fire is only warm when it exits then you can do one burn a day and it would slowly give out the heat it collected the rest of the day. Only, instead of heating a house for an entire day from a single burn I would be warming water from a single burn. I could collect brick from demolished buildings and build a brick tank with very think massive structure that has channels in it for the fire flue gasses.

http://www.motherearthnews.com/Do-It-Yourself/1983-11-01/Brick-Stone-Home-Heating.aspx

Just a thought...

That sounds similar to cob furnaces that I've read about. Different heat storage material, but same idea. You can also implement that approach to a rocket stove that has a very high combustion efficiency.

Hmmm...

You could create your pond as a depression in the exit flue so the heat exchange would go combustion gas <-> brick/cob/whatever <-> water.

Expanding the system would be a pain though

Good temp range for Tilapia. Well Blue tilapia or Oreochromis aureus like we have lots of down in Florida are thought to be pretty cold tolerant as far as Tilapia go. My understanding is that if the water temp drops below 53 F, the immune system of the fish will not recover and the fish will probably get sick and die (at least that is what I've been told.)

At a water temp of 70 F or below, they really don't eat or grow very much.

My water temps since I got the tilapia have been between about 75 F and 90 F (the big system between 76 F and 86 F,) at those temps the fish eat a lot and grow fast.

The north side of the dome will lose a lot of heat in the winter. Have you considered a east-west oriented greenhouse with reflective panels you could add to the north wall in winter? In summer they would add nothing as the sun would be too high, in winter they would add a lot of light and heat. So far my test is adding 15degF (8 degC) to a test system.

I would prefer a compact FT to reduce heat loss as I would not want water temp to fluctuate too much daily.

Coupling the FT to soil will tend to keep it cooler than you want, but will help with preventing really fish-killing cooling. That said, mine is above ground and insulated as I am trying for hot water rather than warm air.

I've got to agree with the others on heat FT>air as the R-value of water>air is about .125 or less, but how about heat gain?
air>FT: Hot air rises, so unless you are sucking it from up high and bubbling it through the tank or
running a heat exchanger up there when temps are high, all you get is infrared coming off that
hot air.....not so good.
sunlight>FT or growbeds etc: good possibilities here...heat can be absorbed well through the sides
of the beds, by the earth, etc and released later.

As far as growbed or FT materials goes, stainless or other highly conductive material would be better than plastic, but not by much: 1/2" of wood is about R0.5, 1/16" of steel is near R0, but the biggest thermal barrier is probably the film of air on any surface (...say R0.5 since normal for the inside surface of a wall is .68 and normal for outside surface of a wall is .17). So you might be looking at R0.5 vs R1 from best to worst case. Plastic would be in between.

Soil has an R-value of about 1/ft, so it will act as insulation, but it gets complex and is a bit controversial. The soil temperature at 6' (2 meters) varies with your location....I second the idea that over time, with a large greenhouse, your base soil temp would change, but it would take a while (years? decades?).

(muttering to himself....say 20 yards deep is about 60,000kg of soil-water per yd^2...say about 60k lbs....say .5BTU/lb per degF......assume one keeps surface at 10degF higher than normal average.....it will take 300,000 BTU or 100kw.hr or (at $.2/kw.hr) $20 of power. Hmmmmmmmmmmmmmm. This is for a buried blue barrel or IBC, but will not take much more for a far far larger tank as long as the unheated outside soil is at least 20 yards away.)

(more muttering.....to keep it at temp.....say about R30 over 30k.SF and 10deg F.....loss is about
(BTU/(SF*degF*hr)/(30)*30k*SF*10*degF=10k BTU/hr is about 3kW. At $.2/kw that is $200 each month.)

(of course, I should be using calculus to distribute heat flow, etc more accurately, but am too dang lazy to refresh my mem and do it right. Besides, this will do for order-of-magnitude.....unless I misplaced a decimal...)

finally, for 80degF you need at least 20degF heating, so double (or triple) everything here.